The disclosed subject matter relates generally to an on-chip temperature sensor and, more particularly, to an on-chip temperature sensor that uses reverse bias current of a p-n diode.
Modern semiconductor devices often include millions of transistors operating at a high speed on a single semiconductor substrate or chip. Thus, on-chip power dissipation and temperature are a significant factor that increases as the population of transistors on a single chip continues to escalate. In many single-chip devices, such as processors, different locations on the chip experience different temperatures due to different levels of activities in and around these locations. Excessive heat of the chip leads to lower reliability, increased electro migration, signal integrity variation, parameters change, and even chip damage. Thus, continuous thermal monitoring by on-chip temperature sensors is used to reduce the possibility of thermal damage and to increase reliability of the semiconductor devices.
Due to the increased design complexity, density of VLSI circuits, operating speeds, and in some cases unequal temperature gradient across the chip, there needs to be many of such sensors distributed across the chip to sense the temperatures. Since these sensors do not take part in the main activities of the chip, for example, in the main computing activities of a processor, but rather, play an auxiliary role of temperature monitoring, their presence in terms of area, and power should be minimal. Technology scaling with nanometer-scale devices has brought many advantages to digital circuits, but at the same time has created many design challenges for analog circuits due to lower voltage headroom, less transistor gain due to short channel effects, increased offset and leakage. These challenges have sometimes become a motivating reason to design digitally assisted high precision mixed-signal circuits.
Various temperature sensing circuitry has been utilized in the past. For example, some designs have used a difference between the base-emitter voltages of a substrate PNP transistor (thermal diode), which is fed by two different currents. However, these sensors require high currents to produce a reasonable amount of voltage to be processed by the subsequent circuits. In some instances, these relatively small voltage need to be amplified before they are processed by precision mixed-signal circuits, for example an Analog to Digital Converter (ADC). Thus these types of sensors tend to consume more power and area. Ultra-low power temperature sensors based on sub-threshold operation of the CMOS transistors have been reported. However in deep sub-micron technologies sub-threshold leakage limits the performance of such sensors. A time-to-digital-converter based on the propagation delay of inverters or ring oscillators based sensors occupy large area and consume excessive power at the required sampling rate.